1. Technical Field
The present invention relates to double-stator rotating electric machines which include a rotor, an outer stator disposed radially outside the rotor, and an inner stator disposed radially inside the rotor.
2. Description of Related Art
Japanese Patent Application Publication No. JP2007261342A discloses an in-wheel motor which includes a rotor and a pair of outer and inner stators. The rotor is connected to a wheel shaft so as to rotate together with the wheel shaft. The outer stator is fixed to a housing so as to be positioned radially outside the rotor with an outer gap formed therebetween. The inner stator is fixed to the housing so as to be positioned radially inside the rotor with an inner gap formed therebetween. That is to say, the in-wheel motor is a double-gap and double-stator motor.
Moreover, in the in-wheel motor, the outer stator includes a plurality of iron cores each having a coil wound thereon. The inner stator includes an iron core having a plurality of protruding pieces; each of the protruding pieces has a coil wound thereon. The rotor includes an annular rotor core, a plurality of outer permanent magnets and a plurality of inner permanent magnets. The rotor core is formed by laminating a plurality of thin steel sheets. The rotor core has a plurality of fitting holes that are formed in a radially outer surface of the rotor core along a circumferential direction of the rotor core. Each of the outer permanent magnets is fitted in one of the fitting holes of the rotor core. Each of the inner permanent magnets is attached on a radially inner surface of the rotor core along the circumferential direction so as to be radially aligned with one of the outer permanent magnets.
With the outer and inner permanent magnets, it is possible to generate a large magnet torque. Moreover, the rotor core has an annular yoke portion that magnetically connects each adjacent pair of magnetic poles formed by the permanent magnets. Therefore, as viewed from the stators, the reluctance of the magnetic flux passage of the rotor is small, i.e., the d-axis (or direct-axis) inductance is large in comparison with the case of a rotor core having no annular yoke portion. However, since the surfaces of the rotor core are occupied by the magnetic poles formed by the permanent magnets, there is almost no space left for the q-axis (or quadrature-axis) magnetic flux to flow through. That is, the q-axis inductance is small. Consequently, the reluctance torque, which increases with decrease in the d-axis inductance and with increase in the q-axis inductance, is accordingly small. As a result, the total torque (i.e., the sum of the magnet torque and the reluctance torque) is small for the size of the motor.
To increase the reluctance torque, one may consider omitting the annular yoke portion from the rotor core. However, in this case, the magnetic flux passages of the outer and inner stators would be serially connected to each other. Moreover, due to the geometric necessity, the magnetic flux passage of the inner stator would be narrower than that of the outer stator. Therefore, the amount of the magnetic flux passing through the outer and inner stators would be limited to a maximum allowable amount which is allowed to pass through the narrower magnetic flux passage of the inner stator. Consequently, the total torque would still be small for the size of the motor.
In addition, in the above in-wheel motor, since the outer permanent magnets are fixed only by being fitted in the fitting holes formed in the radially outer surface of the rotor core, during high-speed rotation of the rotor, it may be difficult to securely retain the outer permanent magnets in the fitting holes against the centrifugal force.